The present invention relates to a monitor circuit for use in anti-skid brake control systems and the like and, more particularly, to such a monitor circuit that energizes an indicator when powered-up or when a fault has been detected in the system. Moreover, the invention is directed to such a monitor circuit that has relatively improved reliability.
Anti-skid brake control systems with which the monitor circuit of the present invention may be used are disclosed, for example, in U.S. Pat. No. 3,917,359 and in copending U.S. Patent Applications Ser. No. 685,267, filed May 11, 1976, for "Anti-Skid Brake Control System with Short Circuit Protection" now U.S. Pat. No. 4,040,676, issued Aug. 9, 1977; Ser. No. 769,255, filed Feb. 16, 1977, for "Anti-Skid Brake Control System with Power-Up Delay"; Ser. No. 770,535 filed Feb. 22, 1977, for "Anti-Skid Brake Control System with Circuit For Monitoring Slower Wheel"; and Ser. No. 779,205, filed Mar. 18, 1977, for "Capacitive Shunt to Minimize Noise Effects in an Anti-Skid Brake Control System". These copending applications are assigned to the same assignee as the present application.
The principal purpose of an anti-skid brake control system is to provide automatic overriding control of the brakes of a vehicle when an incipient or actual skid (hereinafter skid), a locked wheel, or a like condition exists. Upon detecting such a condition, the system dumps, for example, to the ambient environment, part or all (hereinafter a percentage) of the brake operating fluid pressure (hereinafter air pressure) which the vehicle operator then may be attempting to apply manually by pressing on the vehicle brake pedal with his foot. By dumping a percentage of the air pressure the skidding wheels, for example, are permitted to re-gain traction with the road surface. After the skid, locked wheel, or like condition has terminated, the system stops the air pressure dump allowing all of the air pressure requested by the driver to be delivered to the respective air brakes.
In such a system a transducer produces an AC transducer signal that has a frequency indicative of the speed of the vehicle wheel being monitored. A controller is operative in response to the AC transducer signal to produce a dump signal that energizes one or more solenoids in a modulator air brake valve to dump a percentage of the air pressure requested by the driver when the condition or change of the AC transducer signal indicates a skid or locked condition of the wheel. A safe direction failure control circuit monitors various portions of the system and is operative to shut down the system upon detecting a fault. Moreover, a monitor circuit coupled to the safe direction failure control circuit watches the output of the latter and is operative in response to detection of a fault thereby to energize an indicator lamp warning the driver that a fault has been detected in the system and, usually, that the system has been shut down or made inoperative so that full manual control of the vehicle brakes has been restored.
In a vehicle that has a plurality of axles, such as a truck, there may be a separate anti-skid brake control system associated with each respective axle. For example, in a vehicle with two axles respective transducers would be coupled to each of the forward wheels and would deliver AC transducer signals to a common controller which is operative to dump a percentage of the air pressure requested by the driver to be delivered to the brakes of the forward wheels if either of those wheels skids or locks. Respective transducers and a further common controller would be similarly associated with the rear wheels and air brake valves thereof.
A common monitor circuit may be coupled to the respective safe direction failure control circuits of all the controllers or to a common safe direction failure control circuit for plural controllers to provide at least two useful functions. The first or checking function of the monitor circuit is to energize a warning lamp, for example, positioned on the dashboard, each time the system or systems and/or the monitor circuit itself are powered up, i.e. electrical power is supplied thereto, to indicate to the driver that the lamp and the other parts of the system or systems are operable; and the second or warning function is to watch the indicator output signals from the safe direction failure control circuit or circuits to energize the warning lamp whenever a fault is detected to indicate the same to the driver. The latter type energization of the warning lamp may be employed to indicate that manual control of the vehicle brake system or at least one portion thereof, e.g. the one associated with the wheels on one axle, has been returned to the driver.
Intermittent energization of the warning light by the monitor circuit has been found to occur on some vehicles which have the other parts of the anti-skid brake control system functioning properly. It has been determined that transient electrical signals (transients), which commonly occur in most vehicles, particularly in electronic systems thereof, have been the cause of such intermittent energization that provides undesirably false information to the driver. Moreover, since the warning lamp coupled to the indicator output terminals of the monitor circuit must be easily removed for replacement purposes, there is a possibility that a short circuit may occur thereat when the warning lamp is removed and/or replaced; and it has been found in the past that a power surge upon short circuiting may cause damage to one or more of the electrical components of the monitor circuit.